1. Field of the Invention
The present invention relates to a heterojunction transistor and, more particularly, to the improvement in heat resistance, radiation resistance, degree of structural freedom, or the like.
2. Description of the Prior Art
In recent years, a transistor utilizing heterojunction have been noticed. This transistor uses an emitter material which has a wider forbidden band than a base material to thereby increase emitter injection efficiency and decrease base resistance. As a result, it is possible to realize a high-speed and high-gain transistor. In particular, the forbidden band of silicon carbide (2.2 to 3.3 eV) is wider as compared with that of silicon. Taking advantage of this property, a heterojunction transistor having an excellent emitter injection efficiency has been proposed.
A conventional heterojunction transistor which is described in Japanese laid open publication No. SHO 62-216364(P) is shown in FIG. 11. An N-type silicon substrate 12 is provided with a collector contact region 14 formed by ion implantation. An N-type collector region 2 is grown thereon. Ions are implanted into the surface of the collector region 2 to thereby form a P-type base region 4. Next, a .beta. silicon carbide layer is grown using low-pressure chemical vapor depression (LPCVD) method and then an N-type emitter region 6 is formed by ion implantation.
In FIG. 12, there is also shown a conventional heterojunction transistor which is disclosed in Japanese laid open publication No. SHO 63-202962(P). A P-type monocrystal silicon layer 26 acting as base region is formed on an N-type monocrystal silicon carbide layer 24 as emitter region. Further, N-type monocrystal silicon layers 30a, 30b and 30c as collector region are diffused thereon. Reference numerals 28 and 32 designate an emitter contact region and a base contact region, respectively.
In FIG. 13, there is shown an energy band of the transistor thus formed. In the figure, Ec, Ev and Ef are the lower end of a conduction band, the upper end of a valence band and the Fermi potential, respectively. In addition, black dots show electrons and white dots show holes. As known from the figure, the forbidden band in the emitter region is larger than that in the base region, so that emitter injection of holes is suppressed. This causes a base current to be reduced, with the result that the emitter injection efficiency improved. As a result, a high-speed and high-gain transistor can be realized.
However, the conventional heterojunction transistor described above has the following problems.
Silicon, which is used as a material for base and collector regions, is inferior in heat and radiation resistances. This leads to the problem that silicon is not usable over the temperature range of 300.degree. C. or in the presence of a high level of radiation.
Another problem is as follows. In the abovementioned heterojunction transistor, the top layer can be arranged as collector region alone. If top layers (30a, 30b and 30c in FIG. 12) are arranged as emitter region, heterojunction cannot be obtained in the emitter region, making it impossible to improve the emitter injection efficiency.